1. Field of the Invention
The present invention relates generally to medical devices and, more particularly, to implantable medical devices.
2. Related Art
Many medical devices such as pacemakers, defibrillators, circulatory assist devices, cardiac assist and replacement devices, cochlea implants, neuromuscular simulators, biosensors, implantable drug delivery pumps and the like are now designed to be implanted in humans or animals. Because many of these devices require a source of power, inductively coupled transcutaneous energy transfer (TET) systems are coming into increasing use. A TET system may be employed to supplement, replace, or charge an implanted power source, such as a rechargeable battery. Unlike other types of power transfer systems, TET systems provide power to the implanted electrical and/or mechanical device, or recharge an internal power source, without use of a percutaneous lead. Thus, possibilities of infection are reduced and comfort and convenience are increased.
Generally, TET systems include a transcutaneous transformer having an external primary coil operationally aligned with an implanted secondary coil. An external power source is connected to a primary circuit that drives the primary coil to induce alternating current in the secondary coil. This alternating current is converted to direct current by a secondary circuit to provide power to the implanted device or power source. The non-implanted portions of conventional TET systems, including the primary coil and its drive circuitry, are attached externally to the patient, typically by a belt or other fastener or garment.
Implantable medical devices must be carefully designed with respect to both volume and shape in order to minimize the risk of ischemia (shortage of blood supply to tissue) which may eventually lead to necrosis (tissue death). This is particularly true for medical devices that are implanted in subcutaneous tissue. The secondary coil of a TET, for example, may be implanted between the dermis layer of the skin and the subcutaneous tissue. Accordingly, it has generally been recognized that the volume of subcutaneously implanted devices should be a minimum consistent with the functional integrity of the implanted device. In addition to designing implantable devices with a minimal volume, intuitive considerations have led designers to avoid sharp corners on the exterior surfaces of the device. However, it is not uncommon for implanted medical devices to cause eventually ischemia and necrosis of the surrounding tissue. This is especially true of devices that dissipate energy in the form of heat, such as the secondary coil of a TET system.
The invention provides an implantable medical device shaped such that, when implanted, it does not substantially restrict blood flow in the tissue surrounding the implanted device, thereby minimizing the risk of ischemia and necrosis. This enables the device to be implanted near tissue layers that are otherwise susceptible to blood flow restrictions when a device of the same volume is implanted. Blood flow adequate to prevent ischemia and necrosis is maintained by minimizing extravascular tissue pressure caused by the implantation of the device. The device may be implanted at locations that cause a layer of tissue to overlay at least one surface of the implanted device. The overlaying tissue layer may be any combination of cutaneous, muscle or fat tissue.
In one aspect of the invention, an implantable device designed to be implanted within a body is provided. The implantable device includes a body section having an exterior shaped to maintain, when the device is implanted under a layer of tissue, extravascular tissue pressure below intravascular blood pressure in the layer of tissue proximate to the implanted device.
In another aspect of the invention, an implantable device designed to be implanted within a body is provided. The implantable device includes a body section including a curved, tissue-overlaying surface having a radius of curvature of greater than approximately 1 cm.
In one aspect of the invention, a transcutaneous energy transfer system is provided. The system includes a primary coil positionable external of a body and a secondary coil housed in a device implantable under a layer of tissue. The primary coil is designed to induce an alternating current in the secondary coil. The device includes a body section having an exterior shaped to maintain, when the device is implanted under a layer of tissue, extravascular tissue pressure below intravascular blood pressure in the layer of tissue proximate to the implanted device.
Various embodiments of the present invention provide certain advantages and overcome certain drawbacks of the conventional techniques. Not all embodiments of the invention share the same advantages and those that do may not share them under all circumstances. This being said, the present invention provides numerous advantages including the noted advantage of minimizing ischemia. Further features and advantages of the present invention as well as the structure and operation of various embodiments of the present invention are described in detail below with reference to the accompanying drawings.